1. Field of the Invention
The present invention relates to a radio-frequency amplification circuit used for, for example, a transmitter and receiver of radio communication and a bias-voltage supply circuit used for it.
2. Description of the Related Art
With regard to a radio-frequency amplifier used for, for example, satellite communication, ground-based microwave communication, mobile communication and so on, in the case in which a radio-frequency amplification transistor is composed of an NPN bipolar transistor, a radio-frequency signal is applied to its base (input terminal), and a radio-frequency signal after amplification is outputted from its collector. On this occasion, for realizing high efficiency for a wide-range radio-frequency signal level, it is necessary to control the direct current bias voltage supplied to a base of a radio-frequency amplification transistor depending on an input signal level, and for that purpose, a bias-voltage supply circuit is connected to the base of the radio-frequency amplification transistor.
With regard to such a bias-voltage supply circuit, the method of controlling base current of the radio-frequency amplification transistor and deciding electric potential of the input terminal by setting a first NPN bipolar transistor composing a current mirror circuit with a radio-frequency amplification transistor and supplying a constant current to the first NPN bipolar transistor is general.
However, in this method, in the case in which input electric power of a radio-frequency signal increases, when base electric potential of the first NPN bipolar transistor composing the current mirror circuit with the radio-frequency amplification transistor fluctuates widely, rectification is generated by a PN junction (diode) between the base and the emitter of the transistor. That is, if the direct current level of the inputted radio-frequency signal fluctuates widely, when the electric potential is at a high level, this diode is powered on and direct current electric potential of the input signal descends. On the contrary, when the electric potential of the input signal is at a low level, since the diode is inverse-biased, electric potential descent does not arise. Since time-average is taken by this rectification, the electric potential of the input terminal of the radio-frequency signal descends with the increase of signal amplitude, and as a result, the electric power of a signal outputted from the radio-frequency amplifier is saturated and high power output cannot be obtained.
As a remedy, generally, there is known a technique of curbing the electric potential fluctuation of the input terminal of the radio-frequency signal by connecting a second NPN bipolar transistor for compensation between a base of a first NPN bipolar transistor mentioned later and a power supply voltage supply line, which compensates the fluctuation of the base electric potential of a first NPN bipolar transistor composing a current mirror circuit with a radio-frequency amplification transistor (for example, refer to Kokai (unexamined patent publication) No. H11(1999)-68473).
FIG. 7 is a circuit diagram including the composition of a bias circuit described in Kokai No. H11(1999)-68473.
In FIG. 7, a code 100 indicates a bias circuit and a code 200 indicates a radio-frequency amplifier. This bias circuit 100 has a function for compensating a base current of a transistor Q200 automatically in the case in which input electric power of the radio-frequency amplifier 200 increases.
In the radio-frequency amplifier 200 shown in FIG. 7, a code 201 indicates a radio-frequency input terminal, a code 202 indicates a radio-frequency output terminal and a code 203 indicates an electric power supply. Further, Q200 indicates a radio-frequency amplification transistor, C201 indicates a condenser connected between the radio-frequency 201 and the a base of the transistor Q200, C202 indicates a condenser connected between a collector of the transistor Q200 and the radio-frequency output terminal 202 and R203 indicates a resistor connected between a collector of the transistor Q200 and the electric power supply 203. Ibe expresses a base current of the transistor Q200 and Ice expresses a collector current of the transistor Q200.
In the bias circuit 100 shown in FIG. 7, a code 101 indicates an electric power supply and Q100 indicates a first NPN bipolar transistor composing the current mirror circuit with the radio-frequency amplification transistor Q200. Further, the transistor Q101 is a second NPN bipolar transistor for compensating base electric potential of the first bipolar transistor Q100.
Further, in the bias circuit 100 shown in FIG. 7, the transistors Q102 and Q103 are NPN bipolar transistors composing a current mirror circuit making the collector current of the second bipolar transistor Q101 a reference current and deciding collector current of the first NPN bipolar transistor Q100. Further, the resistor R100 is a reference resistor of the current mirror circuit with the transistors Q200 and Q100. Further, Iref is a reference current of the current mirror circuit with the transistor Q200 and Q100. Note that, the resistor R101 is a resistor supplying a bias to the base of the radio-frequency amplification transistor Q200 of the radio-frequency amplifier 200.
In the case in which the electric power of the radio-frequency signal inputted to the radio-frequency input terminal 201 increases, the base current Ibe of the radio-frequency amplification transistor Q200 increases and the collector current Ice of the radio-frequency amplification transistor Q200 increases. Simultaneously, the collector current of the second NPN bipolar transistor also increases, wherein this transistor compensates the base electric potential of the current mirror circuit composed of the radio-frequency amplification transistor Q200 and the first NPN bipolar transistor Q100. The transistors Q102 and Q103 operate as a current mirror circuit using a collector current of the second NPN bipolar transistor Q101 as a reference current. Therefore, in the case in which size ratio of the radio-frequency amplification transistor Q200 and the first NPN bipolar transistor Q100 is defined as N:1, N times current, that is, the current mirror ratio times of the reference current as a collector current of the first NPN bipolar transistor Q100 is applied to the collector of the first NPN bipolar transistor Q100. As a result, even if the base electric potential of the radio-frequency amplification transistor Q200 descends, the base current Ibe can be increased automatically so as to compensate the base current.
As described in Kokai (unexamined patent publication) No. H11(1999)-68473, in the composition of a bias circuit deciding the base current Ibe by a current flowing in the first NPN bipolar transistor Q100 composing the current mirror circuit with the radio-frequency amplification transistor Q200, descent by compensating the amount of descent of the base potential can be prevented and descent of gain produced by it can be prevented.
However, in this composition of related art, all that is achieved is that descent of the base potential is prevented and descent of gain is curbed in the high-power side by descent of the base potential, and what can be achieved is the improvement of the saturation characteristic of the radio-frequency amplification circuit further by shifting the point at which gain descends to the further high-power side.